1. Field of the Invention
present invention relates to a method of separating a thin film device, a method of transferring a thin film device, a thin film device, an active matrix substrate and a liquid crystal display device.
2. Background Art
For example, the production of a liquid crystal display using a thin film transistor (TFT) is carried out through the step of forming the thin film transistor on a substrate by CVD or the like. The step of forming the thin film transistor on the substrate is accompanied by high-temperature treatment, and it is thus necessary to use a substrate made of a material having excellent heat resistance, i.e., high softening point and melting point. Therefore, at present, quartz glass is used as a substrate which can resist a temperature of about 1000xc2x0 C., and heat-resistant glass is used as a substrate which can resist a temperature of about 500xc2x0 C.
As described above, the substrate on which the thin film is mounted must satisfy conditions for producing the thin film device. Namely, the substrate used is determined to necessarily satisfy conditions for producing the device mounted thereon.
However, in consideration of only steps after completion of the substrate on which the thin film device such as TFT is mounted, the above-described substrate is not necessarily preferred.
For example, in cases in which the substrate is passed through the production process accompanied with high-temperature treatment, as described above, a quartz substrate, a heat-resistant glass substrate, or the like is used. However, such a substrate is very expensive, and thus causes an increase in product cost.
A glass substrate also has the property that it is heavy and brittle. In a liquid crystal display used for a portable electronic device such as a palm top computer, a portable telephone, or the like, a substrate is preferably as inexpensive as possible, lightweight, resistant to deformation, and hard to break even by dropping. However, in fact, the glass substrate is heavy and weak against deformation, and has the possibility of breakage by dropping.
Namely, there is a gap between the limits caused by production conditions and the preferable characteristics required for products, thereby causing great difficulties in satisfying both the conditions and characteristics.
Therefore, a technique is proposed in which a thin film device is formed on a first substrate by a conventional process, then separated from the first substrate and transferred to a second substrate. Thus, a separation layer is formed between the first substrate and the thin film device as a layer to be transferred. For example, this separation layer is irradiated with light to separate, from the first substrate, the thin film device as the layer to be transferred, which is then transferred to the second substrate.
As a result of experiments, it was found that in some cases of separating the thin film device from the first substrate, a separation phenomenon does not sufficiently occur in the separation layer only by irradiating the separation layer with light, for example.
As a result of intensive research, it was also found that whether or not the separation phenomenon readily occurs depends upon the properties of the separation layer.
There was also a problem in which the laminate relation of the layer to be transferred to the first substrate used in production of the layer to be transferred differs from the laminate relation of the layer to be transferred to the second substrate to which the layer to be transferred is transferred.
Accordingly, an object of the present invention is to provide a method of separating a thin film device in which before the step of producing a separation phenomenon in a separation layer, the separation layer is securely brought in an easy-to-separate state to accelerate separation of the thin film device from a substrate, and a thin film device, an active matrix substrate and a liquid crystal display device, which use the separation method.
Another object of the present invention is to provide a method of transferring a thin film device which can make the laminate relationship of a layer to be transferred to a substrate used in producing the layer to be transferred coincide with the laminate relationship of the layer to be transferred to a transfer material to which the layer to be transferred is transferred.
(1) The present invention provides a method of separating a thin film device comprising:
the first step of forming a separation layer on a substrate;
the second step of forming a thin film device on the separation layer; and
the third step of producing a separation phenomenon in the separation layer and/or the interface to separate the substrate from the separation layer;
wherein the ion implantation step of implanting ions into the separation layer is provided before the third step.
The separation layer having, for example, the property of absorbing light is provided on the substrate, for example, such as a quartz substrate having high reliability in device manufacture, and the thin film device such as TFT is formed on the substrate. Preferably, the thin film device is then joined to a desired transfer material with, for example, an adhesive layer held therebetween. Then, the separation layer is irradiated with light, for example, to produce a separation phenomenon in the separation layer. As a result, the substrate can be peeled from the substrate, for example, by applying force to the substrate.
At this time, ions are implanted into the separation layer before the separation step to cause the significant separation phenomenon in the separation layer in the separation step, thereby permitting secure separation of the thin film device from the substrate.
In this method, ions are previously implanted into the separation layer to exert the action defined below in any one of (2) to (5), causing the significant separation phenomenon in the separation layer.
(2) The third step preferably includes the step of gasifying the ion implanted into the separation layer. This gasification of the ion in the separation layer causes internal pressure in the separation layer to accelerate the separation phenomenon.
(3) The third step described above in (2) preferably includes the step of irradiating the separation layer with light. This can gasify the separation ion by the light. At this time, irradiation of the substrate from the rear side thereof can decrease the quantity of light incident on the thin film device layer and prevent deterioration in characteristics thereof.
(4) In the ion implantation step, bonds of atoms or molecules which constitute the separation layer are preferably cut by the ions to previously damage the separation layer. This accelerates the separation phenomenon in the separation layer, which is caused in the subsequent separation step.
(5) In the ion implantation step, the characteristics of the separation layer are preferably changed to previously weaken adhesion between the separation layer and the substrate. This facilitates the separation phenomenon in the separation layer, which is caused in the subsequent separation step.
(6) The second step preferably includes the thin film transistor forming step of forming a thin film transistor, the thin film transistor forming step preferably includes a channel layer forming step, and the ion implantation step is preferably performed after the channel layer forming step.
The channel forming step is a high-temperature treatment step, as compared with the other steps. Therefore, if the ions for accelerating the separation phenomenon are implanted before the channel forming step, the ions are possibly released from the separation layer during subsequent high-temperature treatment.
(7) The thin film transistor forming step includes a channel pattern forming step after the channel layer forming step, and the ion implantation step is preferably performed after the channel pattern forming step.
For example, even when the ions for accelerating the separation phenomenon are implanted from the channel pattern side after the channel pattern is formed, the area of the channel pattern itself which interferes with the implantation is decreased. Therefore, the ions can easily be caused to reach the separation layer.
(8) The ion implantation step is preferably performed with the mask formed on a region of the channel layer, which serves as a channel region.
This is because ion implantation in the channel region has the possibility of deteriorating transistor characteristics. The step of implanting the ions with the channel region masked may be performed either before or after the channel pattern is formed.
(9) The thin film transistor forming step includes the step of forming a gate insulation film on the channel pattern and the step of forming a gate electrode on the gate insulation film after the channel pattern forming step, and the ion implantation step is preferably performed by using the gate electrode as the mask.
Since the gate electrode is formed opposite to the channel, the gate electrode can also be used as the mask for preventing ion implantation in the channel region. Another mask may be further formed on the gate electrode according to the acceleration voltage of the ion.
(10) The ion implantation step preferably comprises simultaneously implanting impurity ions to be implanted in at least one of the source region and the drain region of the channel region, and the above-described ions having lower mass and to be implanted in the separation layer.
This enables the step of implanting the ions in the separation layer to be also used as the step of forming the impurity ion in the source and/or drain region. Since the mass of the ions is lower than the impurity ions, the ions can reach the separation layer deeper than the source and drain regions.
(11) The thin film transistor forming step includes the step of forming an amorphous silicon layer as the channel layer, and the crystallization step of crystallizing the amorphous silicon layer by laser annealing, and the ion implantation step is preferably performed before the crystallization step.
If the channel layer is damaged by the ion implantation step, crystallinity can be improved by the subsequent laser annealing step.
(12) The ions are preferably hydrogen ions.
(13) The process temperature in the step carried out after the ion implantation step is preferably less than 350xc2x0 C.
Since hydrogen implanted in the separation layer begins to escape by heating to 350xc2x0 C. or more, the step which requires a process temperature of 350xc2x0 C. or more is preferably performed before the step of implanting the ions in the separation layer.
(14) A thin film device of the present invention is separated from the substrate by the separation method described above in any one of (1) to (13). This thin film device can easily be separated from the separation layer, and thus requires only a low mechanical pressure to be exerted in separation, thereby decreasing defects depending upon the magnitude of the load.
(15) An active matrix substrate of the present invention comprises thin film transistors arranged in a matrix, and pixel electrodes connected to ends of the thin film transistors to form a pixel region, wherein the active matrix substrate is produced by transferring the thin film transistors of the pixel region by using the method described above in any one of (6) to (13).
The active matrix substrate also permits a decrease in defects, as in the invention described above in (13).
(16) A liquid crystal display device of the present invention is manufactured by using the active matrix substrate described above in (15).
Since the liquid crystal display device uses the active matrix substrate described above in (15), defects in the whole liquid crystal display device are also decreased.
(17) A method of transferring a thin film device of the present invention comprises the first step of forming a first separation layer on a substrate, the second step of forming a layer to be transferred, which includes a thin film device, on the first separation layer, the third step of forming a second separation layer comprising a water-soluble or organic solvent-soluble adhesive, the fourth step of joining a primary transfer material to the second separation layer, the fifth step of removing the substrate from the layer to be transferred with the first separation layer as a boundary, the sixth step of joining a secondary transfer material to the lower side of the layer to be transferred, and the seventh step of removing the primary transfer material from the layer to be transferred with the second separation layer as a boundary, wherein the layer to be transferred, which includes the thin film device, is transferred to the second transfer material.
This method comprises removing the first separation layer from the lower side of the layer to be transferred, joining the secondary to the lower side, and then separating the primary transfer material from the layer to be transferred with the second separation layer as a boundary. In this method, the secondary transfer material is present at the initial position of the substrate relative to the layer to be transferred, and thus the initial laminate relation of the layer to be transferred to the substrate coincides with the laminate relation of the layer to be transferred to the secondary transfer material. Since the water-soluble adhesive or organic solvent-soluble adhesive is used for the second separation layer, the primary transfer material can be separated only by bringing the second separation layer into contact with water or an organic solvent.
(18) Another method of transferring a thin film device of the present invention comprises the first step of forming a first separation layer on a substrate, the second step of forming a layer to be transferred, which includes a thin film device, on the first separation layer, the third step of forming a second separation layer, which comprises an adhesive having a separating function due to heating or ultraviolet irradiation, on the layer to be transferred, the fourth step of joining a primary transfer material to the second separation layer, the fifth step of removing the substrate from the layer to be transferred with the first separation layer as a boundary, the sixth step of joining a secondary transfer material to the lower side of the layer to be transferred, and the seventh step of removing the primary transfer material from the layer to be transferred with the second separation layer as a boundary, wherein the layer to be transferred, which includes the thin film device, is transferred to the secondary transfer material.
Namely, as the second separation layer, an adhesive which can be separated by heating or ultraviolet rays is used in place of the adhesive described above in (17).
In this method, the primary transfer material is separated by bringing the second separation layer into contact with the adhesive which can be separated by heating or ultraviolet rays, thereby causing the initial laminate relation of the layer to be transferred to the substrate coincide with the laminate relation of the layer to be transferred to the secondary transfer material.